Generally known brushless motors consist of a permanent magnet rotor which has a plurality of permanent magnets for a field inserted in a yoke made by laminating steel sheets and a stator which has magnetic poles opposing to the outer periphery of magnetic poles of the above permanent magnet rotor with a small space therebetween.
FIG. 35 is a sectional view in a direction intersecting at right angles with the rotatable shaft of a brushless motor using a conventional permanent magnet rotor. In this drawing, a conventional brushless motor 51 consists of a stator 52 and a permanent magnet rotor 53. The stator 52 has the permanent magnet rotor 53 rotatably supported therein and many stator magnetic poles 54 protruded inward. The stator magnetic poles 54 have a coil (not shown) wound thereon. Passing a current through the coil excites a prescribed magnetic pole of the stator magnetic poles 54. A magnetic pole face 55 at the end of the stator magnetic poles 54 is positioned above a cylindrical face at an equal distance from the center of a rotatable shaft 56 of the motor.
The permanent magnet rotor 53 consists of a yoke 57 made by laminating many steel sheets and a pair of permanent magnets 58 for a field. The yoke 57 has four magnetic poles 59 protruded externally on its outer periphery, and the permanent magnets 58 for the field are inserted in every other bases of the magnetic poles 59 with N poles opposed to each other. A magnetic pole face 60 at the end of each magnetic pole 59 is formed to have a curved shape at an equal distance from the center of the rotatable shaft 56, and opposed to the magnetic pole face 55 at an equal distance at every point on the face of the rotatable magnetic pole face 60.
In the above permanent magnet rotor 53, the repulsion of the N poles of the permanent magnets 58 for the field causes the magnetic fluxes to get out of the magnetic pole faces 60 without the permanent magnet for the field as shown in the drawing, to pass through the stator, and to enter the yoke 57 from the magnetic pole faces 60 with the permanent magnet for the field. Accordingly, the magnetic poles having the permanent magnet of the permanent magnet rotor 53 become S pole, and those not having the permanent magnet of the permanent magnet rotor 53 become N pole.
As shown in the drawing, the permanent magnet rotor 53 is rotated by exciting the stator magnetic poles 54, which have been slightly deviated in the rotating direction from the center of the magnetic poles 59 of the permanent magnet rotor 53, to N pole. The permanent magnet rotor 53 is rotated by being attracted to the excited stator magnetic poles 54. Then, the stator magnetic poles 54 which are further displaced with respect to the rotated permanent magnet rotor 53 are excited to N pole. The permanent magnet rotor 53 is further rotated by being attracted to the newly excited stator magnetic poles 54. This procedure is repeated to continuously rotate the permanent magnet rotor 53.
The known conventional brushless motor uses a back electromotive force generated by the rotation of the permanent magnet rotor 53 to determine the position of the above permanent magnet rotor. Specifically, the rotation of the permanent magnet rotor 53 causes the magnetic fluxes of the permanent magnets 58 for the field to cross the coils (not shown) wound on the magnetic pole faces 55 of the stator 52 to generate the back electromotive force in the coils of the stator 52. The position of the back electromotive force is detected to detect the position of each permanent magnet for a field of the permanent magnet rotor 53, and the position of the magnetic poles to be excited on the stator side is determined and excited.
FIG. 36 shows a conventional permanent magnet rotor in an exploded state. A conventional permanent magnet rotor 53 has a yoke 57 and permanent magnets 58 for a field. The yoke 57 is formed by laminating a large number of steel sheets 61. The yoke 57 has magnetic poles 57 formed on the outer periphery, and at the bases of the magnetic poles 59, slots 62 are respectively formed to insert the permanent magnets 58 for the field. Furthermore, each steel sheet 61 is pressed to form caulking sections 63 recessed in the form of a rectangle. The steel sheets 61 are integrally laminated by mutually press-fitting the caulking sections 63.
The permanent magnets 58 for the field are formed to a size capable of being housed in the slots 62. In assembling the permanent magnet rotor 53, an adhesive is applied to the surfaces of the permanent magnets 58 for the field, which are then inserted in the slots 62 with their same magnetic poles opposed to each other as shown in the drawing. Arrows Q in the drawing indicate the directions that the permanent magnets 58 for the field are inserted.
On the other hand, for the permanent magnet rotor 53 which cannot use an adhesive because of its application conditions, the permanent magnets 58 for the field are formed so as to be fitted in the slots 62 without leaving any gap. To assemble the permanent magnet rotor 53, the permanent magnets 58 for the field are pushed in the directions Q shown in the drawing by a pneumatic device so as to be forced into the slots 62. Therefore, a force is applied, in centrifugal directions R, to bridges 64 connecting the leading end of the magnetic pole and the base of the magnetic pole at both ends of the slot.
FIG. 37 shows a permanent magnet rotor in an exploded state developed by the present applicant. It is shown that engagement pawls 62a are formed to protrude to engage with a permanent magnet 58 for a field on the inner periphery of slots 62 for inserting the permanent magnet for the field. The permanent magnet 58 for the field can be inserted in the slots 62, and has a sectional shape to engage with the engagement pawls 62a.
With the above permanent magnet rotor, the permanent magnet 58 for the field is engaged with the engagement pawls 62a only and its frictional resistance is small, allowing to press-fit the permanent magnet 58 for the field into the yoke 57 by a small pressing force. And, when the permanent magnet 58 for the field is press-fitted into the yoke 57, the engagement pawls 62a can hold the permanent magnet 58 for the field to prevent it from coming out.
In the above prior arts, the permanent magnet rotors which apply an adhesive to the outer periphery of the permanent magnets for the field before inserting in the slots of the yoke have disadvantages that the adhesive is dissolved with a refrigerant or pressurizing fluid and the permanent magnets for the field come out.
On the other hand, in the conventional permanent magnet rotor which directly forces the permanent magnets for the field into the slots of the yoke without using an adhesive, a large force is used to press-fit the permanent magnets for the field, and this force sometimes breaks the permanent magnets for the field, or an inserting force is applied to the bridges in the centrifugal directions, possibly resulting in their breakage. And, the above permanent magnet rotor is required to have a high processing precision for fitting the permanent magnets for the field in the slots of the yoke in view of a dimensional tolerance, making it difficult to produce the permanent magnet rotor. Besides, the intimate contact of the permanent magnets for the field with the bridges at both ends of the slots causes the magnetic fluxes of the permanent magnets for the field to leak at the bridges and prevent them from passing the outside space of the magnetic poles, resulting in no cross of the magnetic fluxes with the stator of a motor. Therefore, the magnetic fluxes do not produce a force for rotating the permanent magnet rotor. And, the leakage of the magnetic fluxes at the bridges generates heat due to a core loss.
In view of the above, an object of this invention is to provide a permanent magnet rotor which prevents the permanent magnets for the field from being come out due to a refrigerant or pressurizing fluid, makes positioning of the permanent magnets for the field, can be produced easily, and has high performance.
And, the permanent magnet rotor (see FIG. 37) invented by the applicant has an advantage that a force for press-fitting the permanent magnets for the field is reduced extensively. But, the engagement pawls of each steel sheet are gradually bent in the press-fitting direction when the permanent magnet for the field is press-fitted, this bending of the engagement pawls is accumulated to heavily bend the engagement pawls at the end in the laminating direction of the yoke, and this bending exceeds a binding force of the caulking sections of the steel sheets to partly separate the steel sheets. Besides, in a conventional permanent magnet rotor, because of different tolerances of the permanent magnet for the field and the yoke length in the axial direction, the leading end of the permanent magnet for the field does not completely engage with the engagement pawls of the steel sheets at the end of the yoke when the permanent magnet for the field is shorter than the yoke, resulting in an unstable press-fitted state and sometimes separating the steel sheets due to vibration or the like.
Accordingly, another object of this invention is to remedy the unsolved problems of the permanent magnet rotor invented by the present applicant and to provide a permanent magnet rotor of a brushless motor in which the permanent magnet for the field can be inserted by a small pressing force and prevented from coming out, and the steel sheets at the end of the yoke are not separated when press-fitting the permanent magnet for the field and using, and to provide a method for producing it.
Furthermore, in the above permanent magnet rotor (see FIG. 37) invented by the applicant, part of the magnetic fluxes of the permanent magnet for the field getting out from the N poles passes through the bridges of the yoke to reach the P poles of the permanent magnet for the field. The magnetic fluxes passing through the bridges do not cross the stator of a motor and do not contribute to rotate the permanent magnet rotor. Therefore, the efficiency of the magnetic force of the permanent magnets for the field is lowered in inverse proportion to the magnetic fluxes of the permanent magnets for the field passing through the bridges.
On the other hand, the reduction of the sectional areas of the bridges of the yoke can reduce the number of magnetic fluxes passing through the bridges. This is because the number of magnetic fluxes passing through the bridges is determined from the product of a flux density determined according to the yoke material by a sectional area of the bridges.
But, in the yoke formed by laminating the steel sheets, the steel sheets forming the yoke are generally formed by a punch-out process, but it is quite difficult to punch out the steel sheets for the yoke having the bridges with a very small sectional area. Besides, in the yoke having the bridges with a very small sectional area, the bridges of the yoke are required to have a high mechanical strength because the magnetic poles and the permanent magnets for the field suffer from a centrifugal breakage due to the centrifugal force when the yoke is rotated at a high speed. And when the bridges have a high mechanical strength, there is a disadvantage that the utilization efficiency of the permanent magnets for the field is lowered.
In view of the above, another object of this invention is, in a permanent magnet rotor of a brushless motor having permanent magnets for a field, to provide a permanent magnet rotor which forms a yoke by a plurality of steel sheets laminated, and has an optimum bridge width of the yoke among a width which can be punched out, a width allowable in view of the number of passing magnetic fluxes, and a width allowable in view of a mechanical strength by a centrifugal force.
Besides, in a conventional permanent magnet rotor, the magnetic fluxes of the permanent magnets for the field are concentrated on a position deviated in the rotating direction from the circumferential center of the magnetic poles due to the relation between the bridge width and the width in a radial direction at the magnetic poles, or the relative positional relation of the permanent magnet rotor and the stator of the brushless motor, the back electromotive force generated by the magnetic fluxes is detected earlier than the actual position of the permanent magnets for the field, the magnetic poles of the stator are excited earlier than a prescribed timing, and the permanent magnet rotor has a failure in its rotation.
In view of the above, another object of the invention is to provide a permanent magnet rotor which is formed to concentrate the magnetic fluxes of a magnet for a field to a prescribed position of a magnetic pole and can accurately detect the position of the magnetic pole.